In many countries, the content of nitrates in natural waters has increased and continues to increase due to the extensive use of nitrate fertilizers. Since water containing too many nitrates is not potable, authorities have set maximum permissible levels for the concentration of nitrates in drinking water. For example, the European Economic Community (EEC) has established a directive which sets the desired concentration level of nitrates in potable water at 25 mg/l and the maximum admissible content level at 50 mg/l. Even these concentrations, however, must be considered excessive when the water is to be used in processes, such as in the food industry, in which the product must be concentrated. It is therefore evident that water which is to be used for preparing food must have a very low level of nitrates prior to concentration.
At the present time, several processes are known which allow nitrate removal from water. The known processes include both biological and ion exchange processes, each of which have their disadvantages.
Known biological processes consists of contacting (e.g. in a reactor) the potable water to be denitrated with microorganisms capable of reducing nitrates. See, Denitrification biologique. Usine de Traitment d'Eragny/-Oise (Philipot J. M., Chaffange F., Pascal O., Water Supply (1985) 3:93-98). When using a heterotrophic biological reaction, organic carbon (e.g. acetic acid, methanol or ethanol) must be added to the potable water. These reagents cannot be consumed. They must, therefore, be carefully measured since an accidental excess of organic reagents would render the water unsuitable for consumption. Moreover, the biological reduction reaction leads to the depletion of dissolved oxygen.
The constraints imposed by biological process also lead to costly after-treatment following the biological reaction. Such treatment include filtration to eliminate biomass which is in the water and reoxygenating the water produced. In addition, following a biological process, water may contain bacteria which demands chlorination to disinfect it. Moreover, use of biological processes present a high risk of particularly noxious nitrite contamination which renders water unsuitable for drinking. For example, since the reduction reaction in the biological process is performed in several stages, i.e. NO.sub.3.sup.- .fwdarw.NO.sub.2.sup.- .fwdarw.NO.sub.2 O.fwdarw.N.sub.2, in the event of an incomplete reaction, there is a great risk of nitrites in the water.
Known ion exchange processes use gel or macroporous ion exchangers of an active quaternary ammonium group. The quaternary ammonium group comes from, for example trimethylamine or dimethylethanolamine. See, the "Carix Process" (Hoell W. H., Kretzschmar W., Hagen K., GIT Fachz Lab (1986) 30(4):307-312, 314) and the "Elimination of nitrates by the nitracycle process in the production of potable water" (Deguin A., Eau Industrie et Nuisances (1986) 99:36-40).
These ion exchangers are in the chloride form and thus exchange the chloride ions of the resins against the nitrate ions in the water. The relative affinity of these exchangers are as follows: HCO.sub.3.sup.- &lt;Cl.sup.- &lt;NO.sub.3.sup.- &lt;SO.sub.4.sup.--. It therefore follows that the sulphate ions in the water will be fixed at the same time as the nitrate ions and, consequently, the useful capacity of the exchanger for nitrates will become much weaker as the sulphate concentration in the water increases. A further drawback is that the sulphate and nitrate ions displace the resin's chloride ion which produces water rich in chlorides which can cause corrosion problems in the pipework or, if the chloride content exceeds the advised standard (EEC standard being 200 mg/l chloride), make the water non-potable.
Moreover, the regeneration of exchangers is constantly carried out using large amounts of sodium chloride solution which contributes to pollution. In addition, nitrates fixed by the resin will be freed during the regeneration process and will also be discharged into the environment.
From the foregoing, it can be seen that the use of ion exchange does not change the overall quantity of nitrates discharged into the environment and, therefore, even if the short term problem of producing potable water is resolved, ion exchange does not solve the long term problem of environmental pollution.